If I use Greninja ex's "Shinobi Blade" attack and choose to search my deck for a card, do I have to take a card and put it in my hand if I don't find one I want?
You don't have to search, but if you do you must take a card and put it into your hand.
Source: Twilight Masquerade FAQ; TPCi Rules Team (2024-05-23)

Relevant cards

Note: All applicable cards may not be listed below.
SV6_106
SV6_106